Direct current (DC) power is needed for many types of telephone communication equipment, for control equipment used at electric utility substations and power plants and other similar uses. The DC power may be supplied by a DC power source which may be supplied with alternating current (AC) power from an AC power source, such as the local power grid or a generator and prime mover. Standby batteries are utilized as a backup DC power source when the DC power source either cannot supply all of the power required by the components or when the AC power supply or other external power source is not available, as during a power failure at the local electric utility. The period of time where such battery backup is required may be reduced by providing local diesel electric or turbine powered electric generators. However, during the time where the other backup power sources are unavailable or are being brought into service, batteries are needed. Since the occurrence of power outages is normally infrequent, the condition of the batteries during the times when they are not actively providing the backup power is not known.
A storage battery has an internal impedance, which includes resistive, inductive and capacitive components. When the battery is discharging, DC performance is involved and the resistive component of the impedance is of interest as the discharge current produces a voltage drop across the internal resistance of the battery in accordance with Ohm's law. Over the life of the battery the internal resistance will increase, at a rate determined by such factors as how many times the battery undergoes cycles of discharging and recharging, and other factors. The internal resistance of a battery will eventually increase to a level where the voltage drop across the internal resistance during discharge is so great that the battery can no longer deliver power at its rated capacity. Other defects in the battery or aging of the battery may also result in degradation of the capacity of a battery to perform its function.
When strings of batteries are used to increase the voltage being supplied, or in general when batteries are connected in either series or parallel, the impedance of the overall string has an influence on the amount of energy that can be supplied. Other components of the physical assembly, including connecting links, terminal connections and the like which can exhibit resistance, and whose characteristics may vary with time, due to such factors as corrosion and changes in contact pressure, also contribute to the resultant battery status.